As is well known in the art, trigonometric-based ranging systems are used to obtain data describing the shape of a target object. Conventionally, such system may include one or more structured signal-emitting devices, and one or more optical sensors for sensing the structured signals. A structured signal-emitting assembly includes the structured signal-emitting device(s), and one or more housings fitted with viewports or windows in which the structured signal-emitting device is mounted. Also, a sensor assembly includes the optical sensor(s) and one or more housings fitted with viewports or windows.
Typically, the structured signal is directed to a point, or a number of points, on the target object. If the point or number of points are within a field of view of the sensor, then the position(s) of the point(s) is (are) sensed by the sensor. The location of the sensor relative to the structured signal-emitting device, i.e., their geometric relationship, is known in air, because it can be measured. Conventionally, and as is well known in the art, the geometric relationship is determined in air. The location of the structured signal's intersection with the target object in the sensor's field of view, with the known information regarding the positioning of the structured signal-emitting device and the sensor relative to each other, enables the determination of the location of such intersection. Depending on the structured signal, its intersection with the target object may be in the form of a point or a number of points, e.g., a plane. Based on the measured geometry (i.e., the geometric relationship) between the sensor and the signal-emitting device and the angular transmission and collection properties of the devices in the operating medium, distance measurements are obtained. A large amount of data may be obtained, to result in a 3D cloud, for providing a very accurate image or model of the target object.
For the purposes hereof, a structured signal is considered to be a probe signal with a well-known physical structure that can be used to investigate the layout of an unknown scene (e.g., a target object) by measuring how the structured signal is affected by the scene. An example of a structured signal-emitting device is a laser light plane generator. Where the structured signal is a laser light plane, the optical imaging sensor may be, for example, a charge-coupled device (“CCD”) or an image sensor produced by CMOS (i.e., a CMOS sensor). As is well known in the art, various types of optical imaging sensors may be used, depending on the structured signals that are generated.
To generate data that describes at least a preselected portion of the target object, the structured signal-emitting device and the sensor are movable relative to the target object, or vice versa. For example, the structured signal-emitting assembly and the sensor assembly typically are mounted on a vehicle, above a moving platform (e.g., a conveyor or a rotary stage), or on a rotating frame. The ranging system is then moved relative to the target object or vice versa, and a 3D point cloud is then generated, utilizing a processor. The point cloud is a high resolution, 3D representation of the target object that was scanned. Such systems may be used, for example, for engineering inspections, archeological investigations, and survey work.
A typical trigonometric-based ranging system 10 is schematically illustrated in FIG. 1. As can be seen in FIG. 1, a conventional structured signal-emitting assembly 12 includes a structured signal-emitting device 14 mounted inside a first housing 16 in which a first viewport 18 is positioned. One or more structured signals are emitted from the structured signal-emitting device 12 and transmitted through the first viewport 18 toward a target object 20, as schematically indicated by the reference numeral 21 in FIG. 1. The direction of the transmission is shown in FIG. 1 by arrow “A”. To simplify the illustration, in FIG. 1, the structured signal 21 is shown as intersecting with the target object 20 at a point “P”. As can be seen in FIG. 1, a conventional sensor assembly 22 includes a sensor 24 mounted in a second housing 26 in which a second viewport 28 is positioned. As illustrated in FIG. 1, the point “P” is within a field of view (“FOV”) defined by limits schematically illustrated in FIG. 1 and identified as “L1” and “L2”. It will be understood that the FOV is three-dimensional, centered around an optical axis of the sensor.
The geometric relationship of the structured signal-emitting device 14 and the sensor 24, although defined in three dimensions, can be understood based on FIG. 1. A baseline distance 30 is defined by a straight line or baseline 32 between the structured signal-emitting device 14 and the sensor 24. The sensor 24 and the device 14 each substantially define respective optical axes 34, 36 thereof (It will be understood that the optical axes 34, 36 are not necessarily in the same plane, although they are illustrated in only two dimensions in FIG. 1.) The angles α and β, defined between the optical axis 34 and the baseline 32, and between the optical axis 36 and the baseline 32 respectively, also partly define the geometric relationship between the sensor 24 and the structured signal-emitting device 14. Conventionally, the geometric relationship is determined in air.
As noted above, if the point “P” on the target object 20 is within the FOV of the sensor 24, the structured signal 21 is sensed through the second viewport 28 by the sensor 22. The data thus obtained is processed by a suitable processor (not shown in FIG. 1) to determine the location of the point “P” relative to a coordinate system of the system 10. Typically, data for a large number of points is collected, in order to provide sufficient information to describe the target object 20.
Generally, a trigonometric-based ranging system's data collection properties can vary depending on the medium in which it is operating, thus impacting the ability of the device to accurately interpret the angle at which the signal is being received, i.e., impacting the perceived location of the point “P” in the sensor's field of view. In the prior art, calibration in the environment of operation is required to understand the angular behaviour of the sensor in that environment. As is well known in the art, this angular variation of the receiver is the result of the signals propagating through one environment (optical medium) at a different speed than within another environment (another optical medium).
Accordingly, trigonometric-based ranging systems designed for distance measuring and/or 2D/3D point cloud generation are conventionally calibrated in the optical medium in which they are to be operated. For instance, the trigonometric-based ranging system may be operated in air (i.e., with the target, the structured signal-emitting assembly, and the sensor assembly all located in ambient air), or in water (i.e., with the target, the structured signal-emitting assembly, and the sensor assembly all immersed in water), or in another optical medium. If a trigonometric-based ranging system is to be operated in different optical media (for instance, air and water), separate calibrations of the full system are conventionally required for each of these two sets of calibration properties. For example, a trigonometric-based ranging system intended for use underwater would conventionally be completely immersed for calibration underwater, in a water tank or a pool.
As is well known in the art, the differences between operating in different optical media arise because of refraction at the viewport/optical media interface. Due to refraction, the apparent geometric relationship of the structured signal-emitting device and the sensor is not the same underwater as the measured geometric relationship, i.e., as measured in air. The conventional solution to this problem is to calibrate the entire system in water and in air, e.g., to calibrate in air, and also to immerse the entire system in water in order to determine the apparent geometric relationship in water.
For the purposes of the following discussion, it is understood that the structured signal is a plane of laser light.
In summary, it can be seen that the calibration of the trigonometric-based laser ranging system typically involves two distinct steps: first, determining the optical properties of the sensor and the laser emitter, and determining the spatial relationship between the laser emitter and the sensor. The first of these two steps, the camera calibration, produces a camera matrix based on a pinhole camera model that maps 2D camera sensor positions into 3D unit vectors measured relative to an idealized pinhole (i.e., the principle point of the camera/sensor). See, for instance, Zhang, Zhengyou, “Flexible camera calibration by viewing a plane from unknown orientations”, Proceedings of the International Conference on Computer Vision (ICCV'99), pp. 666-673, IEEE 0-7695-0164-8/99, Kerkyra, Greece, Sep. 20-27, 1999. The second step of the conventional laser scanner calibration process measures the position (i.e., the geometric relationship, apparent and otherwise) of the actual laser light plane in physical units relative to the camera/sensor principle point, in each optical medium of interest.
The first step of the conventional calibration process is specific to the camera alone, but the second step is dependent on the relative position and orientation of the sensor and the laser plane.
This methodology can be extended to other signal and sensor types in that the first stage of calibration is determining, for a particular medium, what the angular model (i.e., the configuration of the field of view) of the sensor is. In the second step, based on this angular model, measurements are made to determine the geometric relationship (apparent and otherwise) between the sensor and the structured signal-emitting device, in each optical medium of interest.
However, the prior art methods have some disadvantages. For instance, in some situations, it is not practical to calibrate a system in its intended operating environment (e.g., both in air and in water), due to cost or safety constraints. That is, calibration of the entire system in water may be difficult, for example, if the system is relatively large. Furthermore, repairs or replacements of individual components of the system normally require a full system re-calibration, i.e., in each optical medium of interest.